In German, commas are used to separate the main clause from the subordinate clause(s).
Er zeigt, = main clause
wie man Eier kocht. = subordinate clause
The subordinating conjunction is wie.
In this sentence there are two clauses separate with a comma
Er zeigt=Main clause (the verb zeigt is in the second position)
Wie man Eier kocht=Subordinate clause (the verb kocht is in the last position).
And also you can change the word order and the verb of the Main clause is always in 2 position (all the subordinate clause is in first position)the verb of subordinate clause is in the last position
Wie man Eier kocht,zeigter